Purification of fused salt electrolytes



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PURIFICATION or r'osnn SALT ELECTROLYTES Bertram C. Haynes, Euclid, Ohio, assignor, by mesne assignments, to Horizons Titanium Corporation, Princeton, N. 3., a corporation of New Jersey No Drawing. Application September 10, 1954, Serial No. 455,341

Claims. (Cl. 204-64) This inventionrelates to the electrolytic deposition of certain transition metals in fused salt baths. More particularly, it relates to a method for purifying the fused salt baths in which these transition metals are electro= deposited.

The usefulness of the so-called transition metals titanium, zirconium, hafnium, vanadium, thorium, tantalum and niobium is, for most purposes, greatly impaired if these metals contain even small amounts of such impurities as oxygen, nitrogen and other metals that tend to embrittle or otherwise adversely affect the quality of the metals produced. The transition metals are commonly prepared by the electrolytic decomposition of a transition metal compound in a fused salt diluent'bath with the resulting deposition of the metal on a cathode immersed in the bath. It has been found, however, that the fused salt bath must be substantially completely free of impurities which would react with or become incorporatedin the transition metal deposited in the cathode in the course of the electrolysis. It is particularly important that the salt bath be free of moisture and of contaminant elements whose oxides or salts are more elect-ropositive than those of the transition metal deposited 'at the cathode. The presence of water or of the aforesaid oxides and salts in the fused salt bath will result in the incorporation of oxygen or other contaminant elements in the transition metal cathode deposit thus impairing the quality and usefulness of the ultimate metal product.

The best commercial grades of transition metal and diluent salt constituents of the fused salt baths employed in the electrolytic production of the aforesaid transition metals contain small but significant amounts of moisture and other deleterious impurities. In particular, the double fluorides of the transition metals, which are a 'common constituent of these fused salt baths, are prone to contain metallic impurities that would contaminate the cathode deposit unless removed from the double fluoride electrolytic cell feed material. Therefore, to avoid the inclusion of oxygen and other impurities in the cathode deposit, it has heretofore been the practice to subject commercial grade cell feed material to extensive and costly purification prior to the electrolysis of the fused salt bath. The purification procedure commonly employed involves wet chemical treatment of the cell feed material to remove therefrom substantially all of the elements that would otherwise contaminate the cathode deposit, followed by careful drying of the purified bath constituents, advantageously by means of a vacuum drying treatment.

To insure that the fused salt bath is completely anhydrous before the electrolytic deposition of the transition metal is initiated, a preliminary electrolysis. of the bath is commonly carried out. The preliminary electrolysis is conducted at voltages and current densities below those at which any appreciable amount of the transition metal constituents of the bath will deposit at the cathode, and is continued until all of the moisture in the bath has been States Patent electrolytically decomposed and removed therefrom. At the relatively low voltages and current densities at which the preliminary electrolysis is carried out, the fused bath is substantially completely dehydrated. However, this relatively mild pro-electrolysis does not remove from the bath any of the contaminant elements whose oxides or salts are more electropositive than those of the transition metal constituent of the bath, necessitating, therefore, the use as cell feed materials of transition metal compounds and diluents salts of substantially absolute chemi cal purity. In' accordance with accepted usage in this art, the term more electropositive hereinafter employed is intended to mean that the voltage required for electrodepositing the impurity ions is higher than or greater than the voltage at which the transition metal is electrodeposited.

l have'now found it possible to use as cell feed material transition metal compounds and diluent salts of commercial grade by purifying these materials in situ in an electrolytic cell. The purification of the cell fee'd material is accomplished by means of a novel electrolytic procedure by which water and contaminant elements whose salts and oxides are more electropositive than those 'of the transition metal are removed from the fused cell feed material as a preliminary to the subsequent electrodeposition of substantially pure transition metal at the cell cathode. Accordingly, my novel preliminary electrolytic procedure for the purification of cell feed material constitutes an important'improvenient in known processes for the electrolytic preparation of the transition metals titanium, zirconium, hafnium, thorium, vanadium, niobiurn and tantalum. The preliminary purification electrolysis of my invention is carried out at a voltage and current density in excess of that required to effect deposition of some of the transition metal at the cathode, and 'sufiicient to effect decomposition of the water and the salts and oxides of the aforesaid contaminant elements and deposition of these contaminan'telements at the cell cathode. This so-called rapid pre ele'ctrolysi's is continued for a period of time suificient to 'insure'that substantially all of the watersand the contaminant elements are removed from the fused salt bath, together with a small amount of the transition metal component of the bath.

When the purification electrolysis is a preliminary step to the subsequent electrolytic deposition of substantially pure transition metal at the cell cathode, I havefou'nd that the voltage employed for the pro-electrolysis should be about equal to that employed for the subsequent electrolysis, and that the current density should be at least about one half that employed for the subsequent electrolysis. Moreover, the passage of about 2 to 5% of the total ampere hours required for substantially complete reduction of the transition metal contents of the fused salt bath will be sufficient to purify fused salt baths composed of cell feed materials of commercialgra'de or better. trolysis procedure can then be electrolyzed in the same electrolytic cell to produce a cathodic transition metal deposit of high purity. Alternatively, the purified bath can be removed from the cell in which the preliminary elec trolysis is carried out for subsequent introduction into one or more other electrolytic cells adapted to carry out the electrolysis by which the desired pure transition metal product is produced.

The fused salt baths to which the process of my invention is particularly applicable are those containing double fluorides of the transition metal as the primary source of the transition metal electrodeposited at the cathode, such as those of the tye described in the copending application of Topinka, McKenna and Carlton, Serial No. 297,158, filed July 3, 1952, which issued- Fused salt baths purified by my rapid pre 'ele'c a January 17, 1956, as United States Patent 2,731,402 of Steinberg and Topinka, Serial No. 262,886, filed December 21, 1951, and of Young and Somerville, Serial Nos. 332,327 and 332,328, filed January 21, 1953, which issued as United States Patents 2,731,405 and 2,731,406 respectively on January 17, 1956. Fused salt baths advantageously purified by my process also include those of the type described in the copending applications of Steinberg, Sibert and Topinka, Serial No. 202,806, filed December 26, 1950, which issued April 26, 1955. as United States Patent 2,707,169 of Wainer, Serial No. 313,795,. filed October 8, 1952, which issued April 26, 1955, as United States Patent 2,707,170, Serial No. 320,113, filed November 12, 1952, which issued as United States Patent 2,722,509 and Serial No. 320,345, filed November 13, 1952, and of Sibert and Burwell, Serial No. 358,194, filed May 28, 1953.

A typical fused salt bath that can be purified by my novel procedure comprises an alkali metal double fluoride of the transition metal, such as potassium fiuotitanate, in admixture with one or more diluent alkali metal or alkaline earth metal halides, such as sodium chloride. The alkali metal double fluorides can be obtained commercially in a reasonably pure condition. However, even the best commercial grades of these salts ordinarily contain appreciable amounts of contaminating metallic oxides and salts, such as the oxides or halides of iron, chromium, aluminum and the like, which must be removed if a pure transition metal deposit is to be produced therefrom. The alkali metal and alkaline earth metal halides can be obtained commercially in substantially pure condition. However, for economic reasons the grades of these salts usually employed contain minor amounts of contaminants that would adversely aitect the quality of the transition metal product. These contaminants can be removed by extensive Wet chemical purification techniques, as described in certain of the aforementioned copending patent applications, to obtain a feed material of a purity satisfactory for fused salt baths used in electrolytic processes for the preparation of pure transition metal deposits. However, by employing the purifying pre-electrolysis procedure of my invention, it is possible to introduce commercial grade salts directly into the fused salt bath without such prior wet chemical treatment designed to remove impurities which can now be removed from the fused salt bath itself pursuant to my process.

In the practice of my invention, the mixture of salts comprising the salt bath is introduced into an electrolytic cell in which an inert atmosphere can be established. Inert atmosphere cells of the type suitable for use in my processes are described in the aforementioned copending applications. In general, such cells comprise a container for the fused salt bath, such as a graphite crucible, disposed in a gastight furnace provided with means for maintaining the fused bath in the crucible at its operating temperature and with means for introducing cell electrodes into the fused bath so that the desired electrolysis can be carried out. It is essential that the electrolysis be conducted under an inert atmosphere. Such an atmosphere may be established by evacuating all of the air and other gases from the interior of the furnace and operating the cell under a vacuum, or by replacing the air in the furnace with an atmosphere composed of an inert gas such as argon.

On establishing an inert atmosphere in the furnace, the mixture of salts in the cell is fused by heating the furnace to above the melting point of these salts. The operating temperature of the cell depends upon the composition of the fused salt bath and upon the requirements of the particular electrolytic procedure employed. Electrodes of a material inert with respect to the molten bath are then inserted in the bath. The cathode is advantageously a graphite rod and the anode may be a 4 similar rod of graphite or, advantageously, it may be the graphite crucible itself.

The electrolysis is carried out at a voltage and current density above that at which the product transition metal will electrodeposit on the cathode. Thus, the preelectrolysis voltage should be about the same magnitude as that at which the subsequent metal deposition electrolysis is carried out, while the current density should be at least about one half that employed for the subsequent electrolysis. For example, when, as in a typical case, the electrolytic production of a transition metal cathode deposit from a fused salt bath requires a voltage in the range of about 5 to 6 volts and a current density in the range of about 450 to 500 amps./dm. the preelectrolysis should be carried at a voltage of about 5 to 6 volts, with a current density of about 200 to 225 amps/dmfi. The passage of about 2 to 5% of the total anticipated ampere hours required for the substantially complete reduction of the initial transition metal con tent of the fused salt bath is ordinarily sufiicient to rid the bath of moisture and all contaminant elements whose oxides and salts are more electropositive than the transition metal constituent of the bath.

The rapid preliminary electrolysis of my invention results in the electrodeposition on the cathode of a small amount of the product transition metal. The transition metal deposited on the cathode during the pre-electrolysis is, of course, contaminated with the metallic elements being removed from the bath therewith and is of little value as a transition metal product per se. However, the loss of this amount of transition metal on the preelectrolysis cathode is not sufiicient to seriously adversely atfect the economies of the overall procedure. Moreover, if desired this preliminary cathode deposit can be recovered and the transition metal component thereof converted into a form suitable for reintroduction into the electrolytic cell.

After the rapid pre-electrolysis has been carried out, the graphite electrode is removed from the bath together with any of the metallic bath impurities and product transition metal that have deposited on the cathode. The purified fused salt bath is then subjected to electrolysis, either in the pie-electrolysis cell or in one or more other electrolytic cells, torecover therefrom the major portion of the transition metal constituent of the salt bath. The product metal recovered from a salt bath purified pursuant to my invention is itself substantially pure and, therefore, is relatively soft and ductile.

The following examples, which compare the transition metal product obtained pursuant to my invention with that obtained when my procedure is not followed, are illustrative but not limitative of the method of my invention:

Example I A mixture of 340 parts by weight of potassium fluotitanate and 1800 parts by weight of sodium chloride were charged to a graphite crucible disposed within an electric furnace. The furnace was equipped with means for establishing an inert atmosphere therein, and with gas-locks for introducing electrodes into the graphite crucible. The potassium fiuotitanate and sodium chloride were of commercial grade and contained small but significant amounts of moisture and metallic contaminants more electropositive than titanium. An inert atmosphere of dry argon gas was established within the furnace and the furnace heated to a temperature of about 800 C. to melt the salts contained in the crucible. After fusion of the salt charge, the temperature of the furnace was maintained at about 800 C. and a inch graphite rod cathode was inserted into the fused salt bath. Employing the graphite crucible as the anode, a direct current was passed through the molten bath to rid the bath of water and metallic contaminants. Dur- '5 ins the ra id ere-elec lys s overall. se vo t e maintained within the rangefof about 4 to 4.5.vlts and the current was about 1.00 amperes. After one-half hour, the electrolysis was stopped and the graphite cathode removed from the bath. The cathode was coated with a thin layer of fine particles of electrodeposited metal. A 1 inch steel rod cathode was then inserted into the bath and electrolysis carried out at substantially the same voltage as previously used and at a current of about 200 amperes. After about hours the bath had become extensively depleted in titanium content, the titanium being electrodeposited on the cathode. in the form of adherent crystals of titanium. The titanium; metal deposited on the steel cathode, after consolidation by melting under an insert atmosphere, had a Rockwell B hardness of 69 and a Brinell hardness number of about 140.

Example 11 A bath identical in its constituents, proportions and grade as that employed in. Example I was prepared and was melted in an inert atmosphere furnace as in Example I. A steel cathode was inserted into the molten bath and a preliminary electrolysis at a voltage of about 1.5 to 2 volts was performed to dehydrate the bath. There was no preliminary electrolysis performed to rid the bath of contaminating metal constituents pursuant to my invention. The metal deposition electrolysis was then performed at about 4 to 4.5 volts and with a current of about 200 amperes. The titanium metal product recovered at the cathode had, after consolidation, a Rockwell B hardness of 89 and a Brinell hardness number of about 180.

Example III A mixture of 600 grams of commercial grade potassium fluozirconate and 1800 grams of commercial grade sodium chloride was introduced into a graphite crucible disposed within the electric furnace employed in Exam,- ple I. The salt charge was melted under an inert atmosphere and the temperature of the molten bath maintained at 750 C. A /2 inch graphite rod was introduced into the molten bath and electrolysis initiated by passing a direct current from the graphite crucible to the graphite rod. The over-all cell voltage was within the range of 5 to 6 volts and the current density was about 200 amps/dm After 45 ampere hours of direct current had been passed through the bath, the graphite rod was removed therefrom and a steel rod was inserted in its place. Electrolysis of the bath was continued at a. volt age within the range of 5 to 6 volts, with a current density of about 450 amps/rim The metallic deposit adhering to the graphite rod was found to have a Rockwell A hardness of 62 while the cast zirconium ingot prepared from the zirconium metal electrodeposited on the steel rod had a Rockwell B hardness of 76.

Example IV A bath identical in composition, proportion and grade to that employed in Example III was prepared and the bath melted under an inert atmosphere in the furnace of Example I. No preliminary electrolysis was carried out to rid the bath of metallic contaminants. A steel cathode was inserted into the bath and electrolysis was carried out at a voltage and current density of about 5 to 6 volts and 450 amps./dm. respectively. On completion of the electrolysis, the steel rod was removed with its adhering deposit of zirconium metal. The zirconium metal was consolidated under an inert atmosphere and found to have a Rockwell A hardness of 56.

1 claim:

1. In the electrolytic deposition of a transition metal of the group consisting of titanium, zirconium, hafnium, thorium, vanadium, niobium and tantalum in a fused salt bath, the improvement which comprises removing water and contaminant elements whose salts and oxides are more eleetropositive than those of said transition metal from, the fused salt bath by subjecting said bath to a preliminary electrolysis at a cell voltage in excessof that required to effect deposition of the transition metal at the cathode. andsuffieient to. effect decomposition of said water andsaid salts and oxides of said contaminant elements and, deposition of said contaminant elements at the cathode, and continuing said preliminary electrolysis for a period'of time sufiicient to insure substantially complete removal of all of said water and said contaminant elements from said bath thereby removing said contaminant elements from the bath and subsequently electrolyzing the purifiedbath to obtain the transition metal in substantially pure form as, an electrodeposit.

' 2. In the electrolytic deposition of a transition metal ofthe groupconsisting oftitanium, zirconium, hafnium, thorium, vanadium, niobium and tantalum in a fused salt bath containing a transition metal compound and at least one diluent salt of the group consisting of alkali metal: andallgaline earth; metal halides, the improvement which comprises removing water and contaminant elements whose salts and oxides are more electropositive than those of said transition metal from the fused salt bath by subjecting said bath to a preliminary electrolysis at a cell voltage in excess of that required to eifect deposition of the transition metal at the cathode and sufiicient to efiect decomposition of said water and said salts and oxides of said contaminant elements and to effect deposition of said contaminant elements at the cathode, and continuing said preliminary electrolysis for a period of time sufficient to insure substantally complete removal of all of said water and said contaminant elements from said bath thereby removing said contaminant elements from the bath and subsequently electrolyzinlg the purified bath to obtain the transition metal in substantially pure form as an electrodeposit.

3. In the electrolytic deposition of a transition metal of the group consisting of titanium, zirconium, hafnium, thorium, vanadium, niobium and tantalum in a fused salt bath comprising a halide of the transition metal and at least one diluent metal salt of the group consisting of alkali metal and alkaline earth metal halides, the improvement which comprises removing water and contaminant elements whose salts and oxides are more electropositive than those of said transition metal from the fused salt bath by subjecting said bath to a preliminary electrolysis at a cell voltage in excess of that required to etlect deposition of said contaminant elements at the cathand suflicient to eifect decomposition of said water and said salts and oxides of said contaminant elements and to effect deposition of said contaminant elements at the cathode, and continuing said preliminary electrolysis for a period of time sufiicient to insure that substantially all of said water and said contaminant elements are removed from said bath thereby removing said contaminant elements from the bath and subsequently electrolyzing the purified bath to obtain the transition metal in substantially pure form as an electrodeposit.

4. In the electrolytic deposition of titanium metal in a fused salt bath containing a titanium compound and at least one diluent metal salt of the group consisting of alkali metal and alkaline earth metal halides, the improvement which comprises removing water and contaminant elements whose salts and oxides are more electropositive than those of titanium from the fused salt bath by subjecting said bath to a preliminary electrolysis at a cell voltage in excess of that required to effect deposition of titanium metal at the cathode and rsufficient to effect decomposition of said water and said salts and oxides of said contaminant elements and to effect deposition of said contaminant elements at the cathode, and continuing said preliminary electrolysis for a period of time suificient to insure that substantially all of said water and said contaminant elements are removed from said bath thereby removing said contaminant elements from 7 the bath and subsequently electrolyzing the purified bath to obtain the titanium in substantially pure form as an electrodeposit.

5. In the electrolytic deposition of a transition metal of the group consisting of titanium, zirconium, hafnium, thorium, vanadium, niobium and tantalum in a fused salt bath, the improvement which comprises removing water and contaminant elements Whose salts and oxides are more electropositive than those of said transition metal from the fused salt bath by subjecting said bath to a preliminary electrolysis at a cell voltage in excess of that required to effect deposition of the transition metal at the cathode and sutficient to eifect decomposition of said water. and said salts and oxides of said contaminant elements and deposition of said contaminant elements at the cathode, continuing said preliminary electrolysis for a period of time sufficient to insure susbtantially complete removal of all of said water and said contaminant elements from said bath for at least a time suflicient to provide between about 2% and about 5% of the total 20 8 ampere hours required for substantially complete deposition of the transition metal content of the fused salt bath, and thereby removing said impurity-containing cathode deposit from the fused bath and subsequently elcctrolyz ing the purified bath to obtain the transition metal in substantially pure form.

References Cited in the file of this patent UNITED STATES PATENTS 1,524,268 McNitt Jan. 27, 1925 2,311,257 Sawyer et a1 Feb. 16, 1943 2,731,402 Topinka et al Ian. 17, 1956 OTHER REFERENCES Journal of the Electrochemical Society, vol. 101, No. 2 (Feb. 1954), pages 63-78, paper by Steinberg et 211.

Principles of Electroplating and Electroforming by Blum et al., 3rd edition (1949), page 45. 

1. IN THE ELECTROLYTIC DEPOSITION OF A TRANSITION METAL OF THE GROUP CONSISTING OF TITANIUM, ZIRCONIUM, HAFNIUM, THORIUM, VANADIUM, NIOBIUM AND TANTALUM IN A FUSED SALT BATH, THE IMPROVEMENT WHICH COMPRISES REMOVING WATER AND CONTAMINANT ELEMENTS WHOSE SALTS AND OXIDES ARE MORE ELECTROPOSITIVE THAN THOSE OF SAID TRANSITION METAL FROM THE FUSED SALT BATH BY SUBJECTING SAID BATH TO A PRELIMINARY ELECTROLYSIS AT A CELL VOLTAGE IN EXCESS OF THAT REQUIRED TO EFFECT DEPOSITION OF THE TRANSITION METAL AT THE CATHODE AND SUFFICIENT TO EFFECT DECOMPOSITION OF SAID WATER AND SAID SALTS AND OXIDES OF SAID CONTAMINANT ELEMENTS AND DEPOSITION OF SAID CONTAMINANT ELEMENTS AT THE CATHODE, AND CONTINUING SAID PRELIMINARY ELECTROLYSIS FOR A PERIOD OF TIME SUFFICIENT TO INSURE SUBSTANTIALLY COMPLETE REMOVAL OF ALL OF SAID WATER AND SAID CONTAMINANT ELEMENTS FROM SAID BATH THEREBY REMOVING SAID CONTAMINANT ELEMENTS FROM THE BATH AND SUBSEQUENTLY ELECTROLYZING THE PURIFIED BATH TO OBTAIN THE TRANSITION METAL IN SUBSTANTIALLY PURE FORM AS AN ELECTRODEPOSIT. 